Hank calls in a friend to do his push ups for him today to explain how skeletal muscles work together to create and reverse movements. Hank and Claire also demonstrate the role size plays in motor units, the three phase cycle of muscle twitches, and how the strength and frequency of an impulse affects the strength and duration of a contraction. This episode also explains twitch summation, tetanus, and isotonic vs. isometric movements.

Table of Contents
Skeletal Muscles Work Together to Create and Reverse Movements 1:14
Motor Units 3:52
Three Phases of Muscle Twitches 4:41
Strength and Frequency of Impulses 5:29
Strength and Duration of a Contraction 5:29
Twitch Summation vs Tetanus 6:19
Isotonic vs Isometric Movements 8:50

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1. Introduction to Anatomy & Physiology
2. Tissues, Part 1
3. Tissues, Part 2 - Epithelial Tissue
4. Tissues, Part 3 - Connective Tissues
5. Tissues, Part 4 - Types of Connective Tissues
6. The Integumentary System, Part 1: Skin Deep
7. The Integumentary System, Part 2: Skin Deeper
8. The Nervous System, Part 1
9. The Nervous System, Part 2: Action! Potential!
10. The Nervous System, Part 3: Synapses!
11. Central Nervous System
12. Peripheral Nervous System
13. Autonomic Nervous System
14. Sympathetic Nervous System
15. Parasympathetic Nervous System
16. Taste & Smell
17. Hearing & Balance
18. Vision
19. The Skeletal System
20. Joints
21. Muscles, Part 1 - Muscle Cells
22. Muscles, Part 2 - Organismal Level
23. Endocrine System, Part 1 - Glands & Hormones
24. Endocrine System, Part 2 - Hormone Cascades
25. The Heart, Part 1 - Under Pressure
26. The Heart, Part 2 - Heart Throbs
27. Blood Vessels, Part 1 - Form and Function
28. Blood Vessels, Part 2
29. Blood, Part 1 - True Blood
30. Blood, Part 2 - There Will Be Blood
31. Respiratory System, Part 1
32. Respiratory System, Part 2
33. Digestive System, Part 1
34. Digestive System, Part 2
35. Digestive System, Part 3
36. Metabolism & Nutrition, Part 1
37. Metabolism & Nutrition, Part 2
38. Urinary System, Part 1
39. Urinary System, Part 2
40. Reproductive System, Part 1: Female Reproductive System
41. Reproductive System, Part 2: Male Reproductive System
42. Reproductive System, Part 3: Sex & Fertilization
43. Reproductive System, Part 4: Pregnancy & Development
44. Lymphatic System
45. Immune System, Part 1
46. Immune System, Part 2
47. Immune System, Part 3

In this Crash Course series, Hank Green teaches most of what you should know about anatomy and physiology, perfect for a freshman-level biology course, or just your own curiosity about the human body.